Composition for preservation of diagnostic test reagents

ABSTRACT

The present invention is concerned with a combination for the preservation of diagnostic tests, characterized by a content of at least two components selected from the group comprising 2-methyl-4-isothiazolin-3-one hydrochloride, 2-hydroxypyridine-N-oxide, chloroacetamide, (N,N-methylene-bis-(N-(1-hydroxymethyl-2,5-dioxo-4-imidazolidinyl))-urea and 5-bromo-5-nitro-1,3-dioxan. 
     The present invention is also concerned with a preserved diagnostic test kit, comprising test reagents and at least two preservation agents selected from the group comprising 2-methyl-4-isothiazolin-3-one hydrochloride, 2-hydroxypyridine-N-oxide, chloroacetamide, (N,N-methylene-bis-(N-(1-hydroxymethyl)-2,5-dioxo-4-imidazolidinyl))-urea and 5-bromo-5-nitro-1,3-dioxan.

The present invention is concerned with the preservation of diagnostic tests and especially of test kits by means of a combination of preservation agents, as well as with diagnostic tests which contain such agents.

Biological and especially diagnostic test kits usually contain, as reagents and as standards, substances which can be decomposed by micro-organisms. For the improvement of their storage stability, they must, therefore, be preserved by the manufacturer. However, hitherto it has proved to be extremely difficult to find appropriate preservatives for such diagnostics since, besides a sufficient effectiveness against bacteria, yeasts and fungi, the reactivity of the substances contained in the test must not be destroyed by the preservation agent. The sensitivity of proteins, for example enzymes or antibodies, as well as of antigens and substrates, towards preservation agents, is manifested, for example, in that the proteins can be denatured or the immunogenic structures are also destroyed. Furthermore, added preservation agents frequently bind such substances and thereby represent a competition for the actual binding component in the test (for example binding of enzyme/substrate, antibody/antigen). Therefore, hitherto it was necessary to find for a particular diagnostic test in each case a preservation agent which was precisely suitable therefor and which possesses the above-mentioned properties.

It is an object of the present invention to provide preservation agents which can be used quite generally in diagnostic tests, cover in their preserving properties a large spectrum of micro-organisms and minimise the formation of resistant micro-organisms. In this way, the testing and storage behaviour of preservation agents is to be limited to a few substances and the search for new preservation agents which display the above-described properties is to be reduced and dropped completely.

According to the present invention, this task is solved by a combination of preservation agents which are, in particular, suitable for the preservation of diagnostic tests and which have a content of at least two components selected from the group consisting of 2-methyl-4-isothiazolin-3-one hydrochloride, 2-hydroxypyridine-N-oxide, chloroacetamide, (N,N-methylene-bis-(N-(1-hydroxymethyl)-2,5-dioxo-4-imidazolidinyl))-urea and 5-bromo-5-nitro-1,3-dioxan.

Surprisingly, we have been able to show that the combination according to the present invention of at least two of the above-mentioned substances acts against a large spectrum of the most varied micro-organisms not only biocidally but thereby also, at the same time, shows no negative influence on the results of the diagnostic test process.

It is especially preferred to use 2-methyl-4-isothiazolin-3-one hydrochloride in combination with at least one of the above-mentioned preservation agents. Therefore, preservation agents are especially preferred which contain 2-methyl-4-isothiazolin-3-one hydrochloride and 2-hydroxypyridine-N-oxide or 2-methyl-4-isothiazolin-3-one hydrochloride and chloroacetamide or 2-methyl-4-isothiazolin-3-one hydrochloride and N,N-methylene-bis-(N-(1-hydroxymethyl)-2,5-dioxo-4-imidazolidinyl))-urea or 2-methyl-4-isothiazolin-3-one hydrochloride and 5-bromo-5-nitro-1,3-dioxan.

It is also preferred to combine 5-bromo-5-nitro-1,3-dioxan with, in each case, one of the other mentioned preservation agents.

2-Methyl-4-isothiazolin-3-one hydrochloride (hereinafter also designated methylisothiazoline) can be obtained by processes such as are described, for example, by S. N. Lewis et al., J. Heterocycl. Chem., 8, 571/1971 and in U.S. Pat. No. 4,105,431 (1968), hereby incorporated by reference. 2-Hydroxypyridine-N-oxide is marketed as a 20% solution under the name Oxy-pyrion by Pyrion-Chemie, Neuss, Federal Republic of Germany and can be isolated therefrom by extraction with ethyl acetate and/or chloroform and subsequent removal of the extraction agent by distillation. (N,N-Methylene-bis-(N-(1-hydroxymethyl)-2,5-dioxo-4-imidazolidinyl))-urea is marketed under the Trade Name Germall 115 by Chemag AG, Frankfurt, Federal Republic of Germany. 2-Chloroacetamide is obtainable from E. Merck, Darmstadt, Federal Republic of Germany. 5-Bromo-5-nitro-1,3-dioxan is marketed by Henkel KG, Federal Republic of Germany under the Trade Name Bronidox.

The anti-microbial effectiveness of these substances is known for individual and several micro-organisms. However, it is surprising that a combination thereof quite generally does not impair the highly sensitive reaction mechanisms such as take place in the case of biological and especially of diagnostic test processes.

Therefore, the present invention also provides diagnostic tests and especially diagnostic test kits which, as preservation agent, contain a combination of at least two substances selected from the group consisting of 2-methyl-4-isothiazolin-3-one hydrochloride, 2-hydroxypyridine-N-oxide, chloroacetamide and (N,N-methylene-bis-(N-(1-hydroxymethyl)-2,5-dioxo-4-imidazolidinyl))-urea and 5-bromo-5-nitro-1,3-dioxan.

The diagnostic test kits according to the present invention can contain the preservation agent in aqueous buffered solution or the preservation agent, together with the test reagents, can be in dry form.

In the tests preserved according to the present invention, 2-methyl-4-isothiazolin-3-one hydrochloride is preferably present in an amount of from 0.001 to 0.5%, 2-hydroxypyridine-N-oxide, chloroacetamide and (N,N-methylene-bis-(N-(1-hydroxymethyl)-2,5-dioxo-4-imidazolidinyl))-urea in an amount of from 0.1 to 0.5% and 5-bromo-5-nitro-1,3-dioxan in an amount of from 0.001 to 0.5%, referred to the concentration in the aqueous solution in which the test kit or the standard or buffer solutions are stored until used. We have found that the preservation agent combination according to the present invention in the test kit can be added not only to the buffer but also to the test reagent in question. However, it has also proved to be expedient simultaneously to add the preservation agent combination to the test reagents and to the buffer solutions. In the case of the test reagents in the test kit according to the present invention, these can be proteins, substrates, hormones, antigens and/or haptens and are usually biologically-active proteins and especially enzymes and/or antibodies and/or protein hormones, for example T₃ or T₄.

The present invention is also concerned with the use of the preservation agent combinations according to the present invention for the preservation of diagnostic tests.

The preservation agent combination according to the present invention has proved to be quite especially suitable for those tests which contain proteins, especially enzymes, antibodies and/or protein hormones, substrates for such enzymes, hormones, especially T₃, T₄ and steroids and/or antigens or haptens.

The following Examples are given for the purpose of illustrating the present invention:

EXAMPLE 1 1.1 Preparation of a mixed suspension of "bacteria"

Escherichia coli ATCC 8739, Pseudomonas aeruginosa ATCC 9027 and Staphylococcus aureus ATCC 6538 P are coated by means of an inoculation loop on to CaSo agar and cultured for 16 to 24 hours at 31° C. From the zone of the inoculation coating where confluent growth passes over into individual colonies, bacterial material originating from at least 5 colonies is removed by means of inoculation loops and suspended homogeneously in about 10 ml. of physiological saline. The suspension thus obtained is designated as test micro-organism suspension T.

The suspension T is so diluted with physiological saline that 10 ml. of inoculum suspension result with a colony count of 10⁷ KBE/ml. A spore suspension in distilled water of Bacillus subtilis is kept on hand, the titre of which is freshly determined every three months. For the preparation of the mixed suspension of "bacteria", the individual inoculum suspensions are mixed in equal parts. To this mixed suspension is added so much of spore suspension of Bacillus subtilis that 10⁵ spores per ml. result. This suspension is designated as mixed suspension of "bacteria".

2.2. Mixed suspension of "mould fungi and yeasts"

Candida albicans ATCC 10231, Rhodotorula rubra DSM 70403, Aspergillus oryzae DSM 6303, Mucor racemosis ATCC 7935 and Penicillium frequentans DSM 62843 are inoculated on to tilt agar test tubes containing Sabouraud maltose agar and cultured for 12 to 16 days at 23° C. The cultures are covered with about 2 ml. of physiological saline and slurried with the help of a sterile glass rod. The suspension which thus results is so diluted with sterile physiological saline that the turbidity corresponds to the McFarland standard 10⁸. This suspension is diluted 1:10 with sterile physiological saline from which results a suspension of about 10⁷ spores or yeast cells per ml. This suspension is designated as mixed suspension of "mould fungi and yeasts".

EXAMPLE 2

The components given in the following Table 1 of commercially available test kit solutions were investigated with the mixed suspensions prepared in Example 1.

Batch A

0.5 ml. of mixed suspension of "bacteria" are added per 50 ml. of the solution to be investigated. The batch thus contains about 10⁵ KBE/ml., the proportion of Bacillus subtilis thereby amounting to about 10³ KBE/ml. The storage temperature of the batch is 31° C.

Batch B

0.5 ml. of the mixed suspension of "mould fungi and yeasts" is added per 50 ml. of the solution to be investigated. The batch thus contains about 10⁵ KBE/ml. of mould fungi and yeasts. The storage temperature of the batch is 23° C.

After a storage period of 6 weeks, the colony count of batches A and B is determined. For this purpose, samples of 0.5 to 2 ml. are removed from the batches and, in two parallel dilution series, diluted with physiological saline corresponding to the expected micro-organism titre and the colony count is determined by plating out on nutrient agar plates. In each case, 0.1 ml. is distributed manually or by means of spiral plating apparatus on to standard agar plates (90 mm. diameter). The dilution depends upon the expected colony count.

The results obtained are given in the following Table 1. The values in this Table state by which number the micro-organisms are reduced by the addition of the preservation agent after a storage period of 6 weeks. The particular order numbers given in the Table indicate the order numbers of Boehringer Mannheim GmbH, Federal Republic of Germany.

The following abbreviations are used in the Tables:

    ______________________________________                                         Brij 35 ® tenside based on polyethylene glycol                                           ethers of lauryl alcohol (manufacturer:                                        Atlas Chemie)                                                    POD           peroxidase                                                       AFP           α-foetal protein                                           CEA           carcinoembryonic antigen                                         FSH           follicle-stimulating hormone                                     HCG           human chorionic gonadotropin                                     LM            luteinising hormone                                              MES           4-morpholine-ethanesulphonic                                                   acid buffer                                                      anti-progesterone-biotin                                                                     conjugate of biotin and an antibody                                            against progesterone                                             PEG 40000/6000                                                                               polyethylene glycol with an average                                            molecular weight of 400000/6000                                  anti-ApoAI    antibody against apolipoprotein AI                               anti-ApoB     antibody against apolipoprotein B                                ______________________________________                                    

                                      TABLE 1                                      __________________________________________________________________________     Micro-organism reduction KBE/ml.                                                          investigated components                                             Test kit   Reagent composition                                                                             bacteria                                                                            mould fungi/yeasts                            __________________________________________________________________________     Cholinesterase Test-                                                                      bottle 1           7 × 10.sup.4                                                                9.6 × 10.sup.4                          Combination                                                                               phosphate buffer 52 mM; pH 7.7                                      order No. 127133                                                                          5,5-dithiobisnitrobenzoate 0.25 mM                                             methylisothiazolone 0.01%                                                      Germall 115 0.01%                                                   Chloinesterase                                                                            bottle 1         6.8 × 10.sup.4                                                                9.2 × 10.sup.4                          System package for                                                                        phosphate buffer 60.7 mM; pH 7.7                                    Hitachi 704                                                                               5,5-dithiobisnitrobenzoate 0.30 mM                                  order No. 851167                                                                          methylisothiazolone 0.001%                                                     Germall 115 0.01%                                                   Albumin automatic                                                                         succinate buffer 750 mM; pH 4.2                                                                 1.6 × 10.sup.4                                                                8.3 × 10.sup.4                          apparatus package                                                                         bromocresol green 1.5 mM                                            order No. 263869                                                                          Brij 35                                                                        2-hydroxypyridine-N-oxide 1.0%                                                 Germall 115                                                         Albumin System                                                                            succinate buffer 75 mM; pH 4.2                                                                  3.6 × 10.sup.4                                                                7.0 × 10.sup.4                          package for                                                                               bromocresol green 0.15 mM                                           Hitachi 704                                                                               Brij 35                                                             order No. 816272                                                                          2-hydroxypyridine-N-oxide 0.1%                                                 Germall 115 0.01%                                                   Saure Phosphatase                                                                         bottle 1           2 × 10.sup.4                                                                1.5 × 10.sup.5                          Test Combination                                                                          citrate buffer 550 mM; pH 4.8                                       order No. 125008                                                                          methylisothiazolone 0.11%                                                      2-hydroxypyridine-N-oxide 1.1%                                      Enzymun-Test AFP                                                                          bottle 1 Incubation buffer                                                                      7.5 × 10.sup.4                                                                1.5 × 10.sup.4                          order No. 711411                                                                          phosphate buffer 40 mM; pH 6.6                                                 methylisothiazolone 0.01%                                                      chloroacetamide 0.1%                                                           bottles 4a-e Standards                                                                          1.1 × 10.sup.5                                                                6.2 × 10.sup.4                                     AFP in human serum                                                             methylisothiazolone 0.1%                                                       2-hydroxypyridine-N-oxide 0.1%                                      Enzymun-Test CEA                                                                          bottle 1 Incubation buffer                                                                      7.3 × 10.sup.4                                                                5.6 × 10.sup.4                          order No. 204501                                                                          acetate buffer 100 mM; pH 5.7                                                  methylisothiazolone 0.01%                                                      chloroacetamide 0.10%                                                          bottles 3a-e Standards                                                                          8.4 × 10.sup.5                                                                2.4 × 10.sup.4                                     CEA in horse serum                                                             methylisothiazolone 0.1%                                                       2-hydroxypyridine-N-oxide 0.1%                                      Enzymun-Test                                                                              bottle 1 Incubation buffer                                                                      7.5  8.6 × 10.sup.4                          Cortisol   phosphate buffer 40 mM; pH 6.0                                      order No. 1096141                                                                         bovine serum albumin 0.30%                                                     methylisothiazolone 0.01%                                                      chloroacetamide 0.10%                                                          bottles 3a-e Standards                                                                          1.1 × 10.sup.5                                                                6.2 × 10.sup.4                                     Cortisol in human serum                                                        methylisothiazolone 0.1%                                                       2-hydroxypyridine-N-oxide 0.1%                                      Enzymun-Test FSH                                                                          bottle 1 Incubation buffer                                                                      7.5 × 10.sup.4                                                                8.6 × 10.sup.4                          order No. 1096923                                                                         phosphate buffer 40 mM; pH 7.4                                                 methylisosthiazolone 0.01%                                                     2-hydroxypyridine-N-oxide 0.1%                                                 bottles 3a-f Standards                                                                          9.4 × 10.sup.4                                                                7.2 × 10.sup.4                                     FSH in bovine serum matrix                                                     methylisothiazolone 0.01%                                                      2-hydroxypyridine-N-oxide 0.1%                                      Enzymun-Test HCG                                                                          bottle 1 Incubation buffer                                                                      7.5 × 10.sup.4                                                                8.6 × 10.sup.4                          order No. 855332                                                                          phosphate buffer 40 mM; pH 7.4                                                 methylisothiazolone 0.01%                                                      chloroacetamide 0.10%                                                          bottles 4a-e Standards                                                                          1.4 × 10.sup.5                                                                6.1 × 10.sup.4                                     HCG in protein matrix                                                          methylisothiazolone 0.05%                                                      2-hydroxypyridine-N-oxide 0.1%                                      Enzymun-Test IgE                                                                          bottle 1 Incubation buffer                                                                      9.6 × 10.sup.4                                                                7.6 × 10.sup.4                          order No. 886882                                                                          Tris-buffer 100 mM; pH 7.6                                                     methylisothiazolone 0.01%                                                      chloroacetamide 0.10%                                                          bottle 2 conjugate buffer                                                      Tris-buffer 100 mM; pH 7.6                                                     bottles 4a-f Standards                                                                          8.4 × 10.sup.5                                                                2.4 × 10.sup.4                                     IgE in horse serum                                                             methylisothiazolone 0.1%                                                       2-hydroxypyridine-N-oxide 0.1%                                      Enzymun-Test                                                                              bottle 1 Incubation buffer                                                                      7.5 × 10.sup.4                                                                8.6 × 10.sup.4                          Insuline   phosphate buffer 40 mM; pH 7.0                                      order No. 974 820                                                                         methylisothiazolone 0.01%                                                      chloroacetamide 0.1%                                                           bottles 3a-e Standards                                                                          9.4 × 10.sup.4                                                                7.2 × 10.sup.4                                     insulin in bovine serum matrix                                                 methylisothiazolone 0.1%                                                       2-hydroxypyridine-N-oxide 0.1%                                      Enzymun-Test LH                                                                           bottle 1 Incubation buffer                                                                      7.5 × 10.sup.4                                                                8.6 × 10.sup.4                          order No. 1096907                                                                         phosphate buffer 40 mM; pH 7.4                                                 methylisothiazolone 0.01%                                                      chloroacetamide 0.1%                                                           bottles 3a-f Standards                                                                          9.4 × 10.sup.4                                                                7.2 × 10.sup.4                                     LH in bovine serum matrix                                                      methylisothiazolone 0.1%                                                       2-hydroxypyridine-N-oxide 0.1%                                      Enzymun-Test                                                                              bottle 1 conjugate buffer                                                                       7.5 × 10.sup.4                                                                8.6 × 10.sup.4                          Progesterone                                                                              phosphate buffer 40 mM; pH 7.0                                      order No. 1204475                                                                         methylisothiazolone 0.01%                                                      chloroacetamide 0.1%                                                           bottle 2 POD conjugate                                                                          6.5 × 10.sup.4                                                                8.1 × 10.sup.4                                     POD ≧ 2 U/ml.                                                           MES 100 mM; pH 5.5                                                             methylisothiazolone 0.01%                                                      chloroacetamide 0.1%                                                           bottle 3 AB-biotin                                                                              6.5 × 10.sup.4                                                                8.1 × 10.sup.4                                     anti-progesterone-biotin                                                       MES 100 mM; pH 5.5                                                             methylisothiazolone 0.01%                                                      chloroacetamide                                                                bottles 4a-e Standards                                                                          1.1 × 10.sup.                                                                 6.2 × 10.sup.4                                     progesterone in human serum                                                    methylisothiazolone 0.1%                                                       2-hydroxypyridine-N-oxide 0.1%                                      Enzymun-Test                                                                              bottle 1 conjugate buffer                                                                       7.5 × 10.sup.4                                                                8.6 × 10.sup.4                          Testosterone                                                                              phosphate buffer 40 mM; pH 7.0                                      order No. 1242024                                                                         methylisothiazolone 0.01%                                                      chloroacetamide 0.1%                                                           bottle 2 POD-conjugate                                                                          6.5 × 10.sup.4                                                                8.1 × 10.sup.4                                     POD ≧ 6.5 U/ml.                                                         MES 100 mM; pH 5.5                                                             methylisothiazolone 0.01%                                                      chloroacetamide 0.1%                                                           bottle 3 AB-biotin                                                                              7.5 × 10.sup.4                                                                8.6 × 10.sup.4                                     anti-testosterone-biotin                                                       phosphate buffer 40 mM; pH 7.0                                                 methylisothiazolone 0.01%                                                      chloroacetamide 0.1%                                                           bottles 4a-e Standards                                                                          9.4 × 10.sup.4                                                                7.2 × 10.sup.4                                     testosterone in bovine serum                                                   matrix                                                                         methylisothiazolone 0.1%                                                       2-hydroxypyridine-N-oxide 0.1%                                      Tinaquant a                                                                               bottle 1         5.3 × 10.sup.4                                                                6.6 × 10.sup.4                          Apolipoprotein AI                                                                         Tris buffer 50 mM; pH 8.0                                           order No. 1174371                                                                         PEG 40000 4.00%                                                                methylisosthiazolone 0.1%                                                      Bronidox 0.10%                                                                 bottle 2         5.3 × 10.sup.4                                                                6.6 × 10.sup.4                                     Tris buffer 100 mM; pH 8.0                                                     anti-ApoAI (sheep)                                                             methylisothiazolone 0.1%                                                       Bronidox 0.10%                                                                 calibrator       5.5 × 10.sup.4                                                                5.4 × 10.sup.4                                     human serum                                                                    methylisothiazolone 0.01%                                                      Bronidox 0.10%                                                      Tinaquant a                                                                               bottle 1         5.3 × 10.sup.4                                                                6.6 × 10.sup.4                          Apolipoprotein B                                                                          Tris buffer 50 mM; pH 8.0                                           order No. 1174380                                                                         PEG 6000 4.00%                                                                 methylisothiazolone 0.01%                                                      Bronidox 0.10%                                                                 bottle 2         5.3 × 10.sup.4                                                                6.6 × 10.sup.4                                     Tris buffer 100 mM; pH 8.0                                                     Anti-ApoB (sheep)                                                              methylisothiazolone 0.1%                                                       Bronidox 0.10%                                                                 calibrator       5.5 × 10.sup.4                                                                5.4 × 10.sup.4                                     human serum                                                                    methylisothiazolone 0.1%                                                       Bronidox 0.10%                                                      Reagent for the                                                                           bottle 1         3.1 × 10.sup.4                                                                5.5 × 10.sup.4                          determination of                                                                          Tris buffer 100 mM; pH 8.0                                          albumin in urine                                                                          PEG 40000 4.00%                                                     (Tinaquanta                                                                               methylisothiazolone 0.1%                                            order No. 1203622)                                                                        Bronidox 0.10%                                                                 bottle 2         3.1 × 10.sup.5                                                                5.5 × 10.sup.4                                     Tris buffer 100 mM; pH 7.2                                                     Anti-hSA (sheep)                                                               methylisothiazolone 0.01%                                                      Bronidox 0.10%                                                                 calibrator       3.1 × 10.sup.5                                                                5.5 × 10.sup.4                                     phosphate buffer 50 mM; pH 8.0                                                 serum albumin 300 mg/l                                                         methylisothiazolone 0.01%                                                      Bronidox 0.10%                                                      __________________________________________________________________________

5. Micro-organism reduction 5.1. Bacteria

The colony count is to decrease in the course of the period of observation by at least 10³ KBE and no significant increase of micro-organisms must occur.

5.2. Mould fungi/yeasts

The colony count is to decrease in the course of the period of observation by at least 10² KBE and no significant increase of micro-organisms must occur.

6. Assessment

The preservation is sufficient when the requirements according to point 5 are fulfilled. In this case, it is to be expected that the protective action of the preservation is sufficient against contamination which occurs in the case of filling under non-sterile conditions or in the case of the usual handling of these reagents (atmospheric microbes, microbes on the hands, non-sterile pipettes and non-sterile packing agents).

7. Influence on the diagnostic test process

None of the combinations used of preservation agents show any influence on the measurement results of the diagnostic tests according to Table 1.

It will be understood that the specification and examples are illustrative but not limitative of the present invention and that other embodiments within the spirit and scope of the invention will suggest themselves to those skilled in the art. 

We claim:
 1. In a diagnostic test kit containing diagnostic test reagents for determination of an analyte the improvement comprising a preservation reagent in said diagnostic test reagents or in a buffer solution, wherein said preservation reagent consists of 2-hydroxypyridine-N-oxide in an active concentration of from 0.1 to 0.5% and at least one other reagent selected from the group consisting of 2-methyl-4-isothiazoline-3-one hydrochloride, chloroacetamide, (N,N-methylene-bis-(N-1-hydroxymethyl)-2,5-dioxo-4-imidazolidinyl))-urea and 5-bromo-5-nitro-1,3-dioxane.
 2. In a diagnostic test kit containing diagnostic test reagents for determination of an analyte the improvement comprising a preservation reagent in said diagnostic test reagents or in a buffer solution, wherein said preservation reagent consists of chloroacetamide in an active concentration of from 0.1 to 0.5% and at least one other reagent selected from the group consisting of 2-methyl-4-isothiazoline-3-one hydrochloride, 2-hydroxypyridine-N-oxide, (N,N-methylene-bis-(N-1-hydroxymethyl)-2,5-dioxo-4-imidazolidinyl))-urea and 5-bromo-5-nitro-1,3-dioxane.
 3. In a diagnostic kit containing diagnostic test reagents for determination of an analyte the improvement comprising a preservation reagent in said diagnostic test reagents or in a buffer solution, wherein said preservation reagent consists of 2-methyl-4-isothiazoline-3-one hydrochloride and chloroacetamide.
 4. In a diagnostic kit containing diagnostic test reagents for determination of an analyte the improvement comprising a preservation reagent in said diagnostic test reagents or in a buffer solution, wherein said preservation reagent consists of 2-methyl-4-isothiazoline-3-one hydrochloride and 2-hydroxypyridine-N-oxide.
 5. In a diagnostic kit containing diagnostic test reagents for determination of an analyte the improvement comprising a preservation reagent in said diagnostic test reagents or in a buffer solution, wherein said preservation reagent consists of 2-hydroxypyridine-N-oxide and (N,N-methylene-bis-(N-1-hydroxymethyl)-2,5-dioxo-4-imidazolidinyl))-urea. 